iwlwifi: mvm: make num_active_macs unsigned

There is no point in having num_active_macs signed
since it should never be negative. Set it to be an
unsigned variable to ensure this.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>